There is no admission that the background art disclosed in this section legally constitutes prior art. Solid phase microextraction (SPME) is a popular solvent-free sampling technique developed by Pawliszyn and co-workers in the early 1990s [1-3]. SPME has gained widespread acceptance and use in laboratories due to the fact that it is a solvent-less extraction technique, its mode of operation is relatively simple and easy to automate, and sampling and sample preparation are combined into one single step.
SPME consists of a fiber that is coated with a stationary phase material, typically composed of a liquid polymer, solid sorbent, or a mixture of both. Equilibrium is established between an analyte and the coating material when the fiber is exposed to a solution, which allows the technique to be applied to both headspace and direct-immersion sampling. When SPME is coupled with gas chromatography (GC), the analytes are desorbed from the fiber coating by thermal desorption in the injection port of the GC.
The development of new coating materials for SPME has flourished in the past decade as the technique continues to gain wide-spread popularity [4-7]. The need for new coating materials is underscored by the fact that SPME methods must achieve high sensitivity and selectivity. The coating material must be designed to be resistant to extreme chemical conditions, such as pH, salts, organic solvents, and modifiers.
To achieve long fiber lifetimes, the coating should be thermally stable to avoid excessive losses during the high temperature desorption step, while also maintaining physical integrity of the film.
As SPME methods become more developed in sampling complicated environmental and biological matrices, structural tunability is a desirable means of modulating specific properties of the coating material while retaining others.
Further, a major challenge facing the world today is the development of a sustainable civilization. An integral component to maintaining sustainability lies with the replacement of polluting processes by benign or “green” alternatives. As industrial practices investigate new green processes, key variables such as cost, feasibility, and significance of improvements are all considerations that influence the adoption of any feasible process.
Description and Properties of Ionic Liquids (ILs)
Ionic liquids (ILs) are a class of compounds that can be tailor synthesized to exhibit unique solvent properties while retaining many green characteristics. Despite widespread interest in ILs, there continue to be many properties of ILs that are not well-understood. Paramount of these properties is how the structures of the cationic and anionic moieties comprising the IL influence the partitioning behavior of various molecules. No single study or collection of studies performed to this date can be used to conclusively predict or explain the role of the IL cation and/or anion on the observed partitioning behavior.
Ionic liquids (IL) and their polymerized analogs constitute a class of non-molecular, ionic solvents with low melting points. Also known as liquid organic, molten, or fused salts, most ILs possess melting points lower than 100° C. [8]. Most widely studied ILs are comprised of bulky, asymmetric N-containing organic cations (e.g., imidazole, pyrrolidine, pyridine) in combination with any wide variety of anions, ranging from simple inorganic ions (e.g., halides) to more complex organic species (e.g., triflate).
ILs have negligible vapor pressures at room temperature, possess a wide range of viscosities, can be custom-synthesized to be miscible or immiscible with water and organic solvents, often have high thermal stability, and are capable of undergoing multiple solvation interactions with many types of molecules. The plethora of interaction capabilities ILs are capable of undergoing include: hydrogen bond acidity, hydrogen bond basicity, π-π, dipolar, and dispersion interactions. These interactions are directly related to the structures of the cationic/anionic moieties that comprise the IL.
The aforementioned properties have made molten organic salts [9-11] and imidazolium and pyrrolidinium-based ILs [12-16] an interesting and useful class of stationary phase materials in GC. In particular, it has been shown that the separation selectivity and thermal stability can be altered by changes to the cation and/or anion, [12-13] polymerization and immobilization of the IL [15], and by blending different ILs to form stationary phases with varied composition [16]. While a series of reports have described the use of ILs in single drop microextraction (SDME) [17-18] and liquid phase microextraction (LPME) [19-21], only two reports have studied the use of ILs in SPME [22-23].
Liu and co-workers reported the development of a disposable IL coating for the headspace extraction of benzene, toluene, ethylbenzene, and xylenes [22]. The resulting fibers possessed comparable recoveries to the commercial fibers coated with polydimethylsiloxane (PDMS).
To allow for a better wetting and increased loading of the IL on the fused silica fiber, Hsieh and co-workers utilized a Nafion membrane followed by dip coating of the SPME fiber in an IL [23]. The fibers were used to extract polycyclic aromatic hydrocarbons (PAHs) from aqueous solution. Using GC-MS, detection limits of around 4-5 ng L−1 were obtained with relative standard deviations ranging from 6-12%. In both of these reports, the IL had to be re-coated on the fiber after each extraction and desorption step, which significantly reduces the convenience and high-throughput nature inherent to SPME.
It has been observed that many classes of neat ILs have a strong propensity to flow off the fiber when employing moderate to high desorption temperatures (200° C. and above) and desorption times of 4 minutes or longer. Several complications arise from the loss of the IL during the desorption step: (1) a compromise between the desorption time and temperature must be achieved; (2) due to the fact that the IL drips into the injection port and contaminates the liner, it must be constantly removed and cleaned to prevent unwanted IL-decomposition products to appear as chromatographic ghost peaks; and, (3) the SPME fiber needs to be re-coated with the IL, thereby making it inconvenient while also decreasing fiber-to-fiber reproducibility.
Due to the negligible vapor pressure inherent to ILs, ILs are not lost at high temperatures and may be recovered and re-used, demonstrating their potential as green solvents. In addition, the implementation of processes using many classes of ILs may minimize the potential for explosions due to the lack of flashpoint and reduced flammability of many ILs. Numerous reports have demonstrated enhanced reaction kinetics and favorable product ratios when performing various organic reactions in an IL instead of traditional organic solvents.
Uses of Ionic Liquids in Analytical Extractions
The initial impetuses for the widespread interest in ILs were organic synthesis and the growth of green chemistry. Research interest in ILs has extended into many fields of science involving an interdisciplinary group of researchers. The numbers of publications examining basic properties and novel applications of ILs have increased over 850% from 2000 to 2006. The study and applications involving ILs in analytical chemistry has been lagging despite the vast opportunities offered by these designer solvents.
In an attempt to better understand the solvation properties of ILs, prior studies have set out to compare the partitioning behavior of neutral, amino-aromatic compounds, and compounds containing mixed acidic and basic functionality in octanol/water and ionic liquid/water systems. While the aforementioned compounds seemed to correlate well between the two systems, a considerable divergence was noted for acidic compounds as well as a strong pH dependence on overall partitioning. Other studies have explored the partitioning of metal ions by task-specific ILs, the use of ILs as extraction media in deep desulfurization of diesel fuels, as well as the use of extractants to remove ions from aqueous solutions.
However, no single study or collection of studies can be used to conclusively predict or explain the role of the IL cation and/or anion on the observed partitioning behavior. Due to recent rapid advances in IL synthesis, it has been proposed that the extensive range of available cations and anions could produce up to 1018 different ILs. A relationship between the structure of ILs and their corresponding physicochemical and solvation properties is desperately needed to intelligently design new classes of ILs for specific applications.
Task-Specific Ionic Liquids
The term “task-specific ionic liquids” (TSILs) relates to salts that incorporate functional groups into one or both of the ions to impart specific interactions with dissolved substrates; e.g., the use of urea, thiourea, and thioether functional groups to remove Hg2+ and Cd2+ from aqueous solutions. In another example, the reactive capture of CO2 was demonstrated by a TSIL containing a tethered amine group. The amine sequesters CO2 through the formation of an ammonium carbamate complex with the TSIL. While many of these elegant compounds have been studied in synthetic reactions and in large scale extraction processes, there has been little work that investigates the incorporation of these compounds into task-specific microextraction devices or applications in other areas of separation science.
Absorbent Coatings for Solid Phase Microextraction and Stir Bar Sorptive Extraction
Solid phase microextraction (SPME) and stir bar sorptive extraction (SBSE) are two solvent-free sampling techniques in which sampling and sample preparation are combined into one single step. SPME consists of a fused silica fiber that is coated with an absorbent or adsorbent coating material, typically polydimethylsiloxane (PDMS), polyacrylate, or carbowax divinylbenzene. Depending on the mode of extraction (headspace or direct immersion), the analytes are sampled due to their partitioning to the coating material, typically under equilibrium conditions. The analytes are desorbed from the fiber using either thermal desorption (i.e., injection port of a gas chromatograph) or by solvent desorption (i.e., solvent chamber coupled to a high performance liquid chromatograph).
SBSE operates in a similar manner to SPME but differs in the type of support and the amount of coating material employed in the extraction. In SBSE, the analytes are extracted into a thick polymer coating on a magnetic stir bar. The amount of coating material in SBSE is ˜50-250 times larger than SPME, which produces a distinct sensitivity enhancement.
Polymer coating materials used in SBSE have largely focused on PDMS, although there has been a report of incorporating sol-gel technology into the PDMS coating material. The development of new coating materials for SPME has flourished in the past five years as the technique has gained wide-spread popularity.
As described above, only two reports have studied the use of ILs in SPME. In both cases, several ILs were chosen and coated on the support to carry out the extraction. Both reports indicated the extraction efficiencies obtained were superior to commercial SPME coating materials. No reports have yet used IL-SPME for the determination of analyte partition coefficients. There have also been no studies reported on the use of TSILs in SPME. Moreover, to the best of the inventors' knowledge, no SPME or SBSE coating material has yet been shown to effectively extract nucleic acids, which has tremendous opportunity in all aspects of bioscience.
There is a need for new coating materials which is underscored by the fact that SPME methods must achieve high sensitivity and selectivity. In addition, the coating material must be designed to be resistant to extreme chemical conditions, such as pH, salts, organic solvents, and modifiers. Additionally, the coating must be thermally stable to maintain physical integrity during the lifetime of the fiber.
There is a further need for ILs that possess the ability to be structurally tuned to effectively meet any physical or chemical requirements.
There is also a need for improved solvent-free sampling techniques. In particular, there is a need for improved the solid phase microextraction (SPME) methods.